In a gas turbine of this type, temperatures which may lie in the range of between 1000° C. and 1400° C. arise in the flow duct after it has been acted upon by hot gas. The platform of the turbine blade, as a result of the annular arrangement of a number of such turbine blades in a blade stage, forms part of the flow duct for a working fluid in the form of hot gas which flows through the gas turbine and thereby drives the axial turbine rotor by the turbine blades. Such high thermal stress on the flow duct boundary formed by the platforms is counter-acted in that a platform is cooled from the rear, that is to say from a turbine blade foot arranged below the platform. For this purpose, the foot and the platform region conventionally have suitable ducting so as to be acted upon by a cooling medium.
An impact-cooling system for a turbine blade of the type initially mentioned may be gathered from DE 2 628 807 A1. In DE 2 628 807 A1, for cooling of the platform, a perforated wall element is arranged upstream of that side of the platform which faces away from the hot gas, i.e. downstream of the platform, that is to say between a blade foot and the platform.
Cooling air under relatively high pressure impinges through the holes of the wall element onto that side of the platform which faces away from the hot gas, with the result that efficient impact cooling is achieved.
EP 1 073 827 B1 discloses a novel way of designing the platform region of cast turbine blades. The platform region is designed as a double platform consisting of two platform walls lying opposite one another. What is achieved thereby is that the platform wall directly exposed to the flow duct and therefore to the hot gas and delimiting the flow duct can be made thin. The design in the form of two platform walls results in functional separation for the platform walls. The platform wall delimiting the flow duct is responsible essentially for the ducting of hot gas. The opposite platform wall not acted upon by the hot gas takes over the absorption of the loads originating from the blade leaf. This functional separation allows the platform wall delimiting the flow duct to be made so thin that the ducting of the hot gas is ensured, without substantial loads in this case having to be absorbed.
In the design of the turbine blade of the type initially mentioned, in a parting plane between platforms of turbine blades of the same blade stage which are contiguous or of adjacent turbine blades of blade stages arranged one behind the other, sealing measures are necessary in order to prevent an unwanted and excessive outflow of cooling medium into the flow duct acted upon by hot gas. The measures required for sealing off may lead to difficult situations in structural and cooling terms on a platform wall subjected to high thermal load and constitute an increased potential for the failure of a turbine blade and consequently of a gas turbine.
Conventionally, the sealing off of such parting planes is achieved by the installation of special sealing elements. However, on the one hand, these have to be sufficiently flexible to permit simultaneous relative movements of adjacent parts, in particular of adjacent turbine blades and their platforms, and, on the other hand, they must nevertheless maintain a sealing action. The installation of such sealing elements leads to geometrically and structurally complicated components. As a result of this, special cooling measures are necessary so that platform edge regions where access is difficult can be cooled sufficiently.
It would be desirable to have a gas turbine in which the boundary of the flow duct is configured as simply as possible and at the same time can be cooled effectively and is sealed off.